High speed differential signal traces are widely used in server/storage product designs. Many server/storage products include a chassis that mounts different printed circuit boards for electronic devices. The printed circuit boards include various signal traces to provide signals to the devices on the boards. Signal traces generally are arranged in differential trace pairs for a particular signal line. Such differential traces on a printed circuit board have different modes; including differential mode, common mode, and mode conversion between differential signals during transmission. Since more and more product applications include differential signal transition between different boards or between boards and cables, common mode energy will radiate through connectors over these transitions to holes in a chassis. Common mode energy results in a signal on both differential traces. Common mode energy can therefore generate noise that interrupts the transmission of signals over the traces and create interference problems.
FIG. 1 is an example of a prior art return current circuit trace 10 on a printed circuit board 12. The printed circuit board 12 is attached to a ground plane layer 14. The current circuit trace 10 includes two differential traces 22 and 24 on one surface 20 of the printed circuit board 12. The ground plane layer 14 contacts the opposite surface of the printed circuit board 12. An arrow 30 shows the insertion current in the differential trace 22. An arrow 32 shows an induction current in the differential trace 24. An arrow 34 shows a return current that is generated in the ground plane layer 14 under the differential trace 22. As shown in FIG. 1 common mode energy is generated by the insertion current represented by the arrow 30 minus coupling terms.
FIG. 2 is a graph of electronic signal interference from signals in a server chassis. The server chassis includes boards with differential traces similar to that shown in FIG. 1. The server chassis has several boards. The transition between the different boards allows common mode energy to radiate through holes in the chassis. A line 50 is the allowed noise for a trace in an FCC Class A digital device while a line 60 is the allowed noise for a system in an FCC Class A-AV device. As may be seen, the allowed noise level is lower for the more modern Class A-AV devices. A spike 80 represents unacceptable noise radiation at approximately 8 GHz frequency generated from an example system such as a server chassis.
To reduce radiation caused by common mode energy, there is a need for trace design that reduces common mode energy, while maintaining total energy for differential signals. There is a further need for a differential trace that allows different shapes of a return path to reduce common mode energy. There is also a need for determining the length of a return path that causes interference to cancel noise at a specific frequency.